1. Field of the Invention
The present invention relates to a four-wheel drive vehicle. In particular, the present invention relates to a control method and a control apparatus for a four-wheel drive vehicle, designed to automatically control torque (driving force) distribution between front and rear wheels according to a running state of the vehicle, and belongs to the field of vehicle running control techniques.
2. Description of the Background Art
In relation to a four-wheel drive vehicle adapted to transmit torque to both front and rear wheels, there have been known various drive systems, such as a full-time four-wheel drive system, and a system capable of switching between two-wheel drive and four-wheel drive based on driver's selection. In addition, recent years, a system capable of automatically controlling torque distribution between front and rear wheels according to a running state of the vehicle has been put into practical use.
In a vehicle employing such an automatic torque distribution system, for example, assuming that the front wheels serve as primary driven wheels and the rear wheels serve as secondary driven wheels, it becomes possible to fundamentally control a drive mode of the vehicle to be set to a two-wheel drive mode for driving only front wheels, and, according to a change in the running state such as an increase in vehicle speed or an increase in steering angle of a steering wheel, controllably distribute torque to the rear wheels. Further, considering torque loss due to slip of the primary driven wheels caused, for example, by a road surface with a low friction coefficient (low μ road), transmission loss of driving force to the secondary driven wheels, etc, the most efficient torque distribution can be realized. In this case, it becomes possible to enhance overall performance including vehicle running performance, steering stability and fuel economy performance.
The torque distribution to the secondary driven wheels is performed using a torque distribution device composed, for example, of an electromagnetic coupling disposed in a driving force transmission path for transmitting torque to the secondary driven wheels therethrough and adapted to be electrically controlled. Specifically, an input current to be applied to the torque distribution device is controlled to control a torque transmission capacity of the device and thereby variably control a torque distribution ratio for the secondary driven wheels according to the running state, for example, within the range of 0 to 50% with respect to the entire torque.
In an electrically-controlled torque distribution device, such as the above electromagnetic coupling, as illustrated in FIG. 7, a characteristic of torque transmission capacity with respect to input current applied thereto is different between when a current value of the input current is increasing as indicated by the curve A and when the current value is decreasing as indicated by the curve B, i.e., a so-called hysteresis occurs. Thus, even if the input current is maintained at the same value I0, the torque transmission capacity varies in the range of T1 to T2 depending on whether the input current is increasing or decreasing.
In this connection, JP 2001-193757A discloses an invention concerning a coupling disposed in a driving force transmission path of a vehicle, which is intended to reduce hysteresis in torque transmission capacity which would otherwise occur due to a frictional resistance in a spline fitting portion, by an improvement in mechanical structure. However, this invention relates to neither a technique for torque distribution between front and rear wheels in a four-wheel drive vehicle, nor a countermeasure against electrical hysteresis in the above torque distribution device.